This invention relates generally to cathode ray tube (CRT) deflection and high voltage circuits and particularly to a shutdown circuit arrangement for rapidly disabling a deflection and high voltage system in the event of a fault condition.
CRT display systems are well known in the art. In transistorized display systems, a horizontal drive supplies a sawtooth shaped current at the deflection frequency to a horizontal output transistor that supplies a load circuit consisting of a deflection transformer. The deflection transformer develops the required horizontal deflection signals which are applied, along with suitable vertical deflection signals, to a yoke structure that is located on the neck of the CRT. These signals control the deflection or scanning of the electron beam produced in the CRT to develop a so-called raster. The horizontal deflection system includes a high voltage winding for developing a high voltage that is rectified and applied to suitable internal electrodes in the CRT for its operation. A source of video information provides the necessary modulating potentials for the CRT electron beam. In some deflection systems, a high voltage "bleed" resistor is used to stabilize the high voltage applied to the CRT.
Many foreign countries have adopted the so-called TUV standard. This standard includes, among its provisions, a specification that the deflection system be disabled in the event of a fault in the bleed resistor that would result in the high voltage electrode being connected to ground. In such a situation, the deflection system must be capable of being disabled quickly enough to avoid damage to its various components.
It has been found that in deflection systems of conventional design, shorting the high voltage electrode to ground does not produce a sufficiently rapid increase in fault current to open the B+ supply fuse or circuit breaker quickly enough to preclude damage to the deflection transformer, sweep transistor or other deflection system components. Accordingly, there is a need to provide a circuit for rapidly disabling the CRT deflection system in the event of a short circuit condition occurring between the high voltage electrode and ground.